1. Field of the Invention
The present invention relates to a radio frequency receiver and particularly, but not exclusively, to an adjustable bandwidth filter for use in a radio frequency receiver for receiving particularly, but not exclusively, digital signals.
2. Description of the Related Art
Communication receivers for receiving digital signals, for example, digitized speech, are well known and frequently have an architecture in which a received signal is applied to an adjustable gain r.f. amplifier prior to a first frequency down conversion stage which produces an i.f. frequency. The i.f. frequency is filtered in, for example, a SAW filter and the filtered signal is applied to another frequency down conversion stage in which the i.f. signal is applied to nominally quadrature related mixers, the outputs of which are low-pass filtered and then applied to a digital signal processor (DSP). FIG. 1 illustrates a typical low-pass filter input-output characteristic in which the ordinate is amplitude in db and the abscissa is frequency f. The wanted signal band is indicated as f.sub.w and the unwanted higher band of frequencies is indicated as f.sub.uw. The receiver noise level is indicated by a horizontal broken line RNL. As is known in the art, in order to be able to determine the correct value of a digital signal, the amplitude of the input signal need only be several dbs above a level termed the Minimum Distinguishable Signal MDS. Accordingly, there is little point in designing filters to handle much bigger signals if the result is the consumption of a lot more current.
If the input signal applied to the low-pass filter should significantly exceed this MDS level, the filter will then be overdriven. Eliminating this problem could lead to an unwanted increase in current consumption due to the filter having to be designed to cope with a dynamic range of input signals which is much larger than is strictly necessary. Generally, this problem is countered by the DSP applying automatic gain control signals to the adjustable gain r.f. amplifier.
In the interests of current saving, the low-pass filters are thus generally designed to handle a limited dynamic range of wanted signals and to consume a low power. However, a problem arises when strong unwanted signals need to be blocked by the filters as there is no AGC applied by the DSP. The filters must be able to cope with such unwanted signals in a linear way. If the filters are designed to be capable of handling large unwanted signals, this could lead to a significant increase in power consumption, for example, 141 times more power would be required for handling an unwanted signal which is greater than the MDS level by 23 db, than for a signal just above that level.
Such low-pass filters are not infrequently voltage driven transconductor filters. However, if such filters comprise current driven transconductor filters then a much larger signal, for example, 14.5 db larger in the case of a Gaussian to 6 db filter, can occur in the stop-band than in the wanted band without the filter going non-linear. Even with this improvement, there is still a desire to cope with an even larger signal, for example, up to 23 db larger, in the stop-band, without the filter going non-linear and also to make the filter have an adjustable bandwidth to be able to selectively pass a narrow band signal which, for example, occurs in the Pan-European or GSM cellular telephone system, and a wider band signal which, for example, occurs in the Digital European Cordless Telephone (DECT) system.